It’s amazing how many people don’t know the main secret to setting up a monitor. The monitor brightness should be set so that it doesn’t contrast with the ambient lighting. That is, if the room is brightly lit, the monitor should be bright too. If the room has dim lighting, the monitor brightness should be lowered as well. You shouldn’t work at a monitor in complete darkness: at least put in a desk lamp, light up the objects behind the monitor.
This has to do with the fact that the eye shouldn’t have to keep readjusting to the brightness of the monitor and the brightness of the surroundings. If this happens very often, your eyes get tired quickly. But if the monitor brightness matches the surroundings, the eye never needs to adjust at all.
Ideally, sit facing a window so you can look into the distance — good conditions for relaxing your eyes.